Re: Writing to a ramdisk in a PV domain is SLLLOOOWWW?!?

[Jeremy Fitzhardinge <jeremy@goop.org>]

On 05/05/2010 11:14 AM, Dan Magenheimer wrote:
> Has anyone tried using a ramdisk in a PV domain?
>
> I'm trying it in an experiment to compare swapping to
> a ramdisk against tmem (frontswap) performance
> (suggested as a performance test on lkml).
>
> Writing to the ramdisk appears to be VERY VERY slow,
> elapsed time in the guest is several times larger than
> user+sys, and xentop shows the guest consuming vcpu
> seconds at about the user+sys rate.  Note that
> this is when tmem is turned off and there is no
> vhd swap disk configured.
>
> I'm suspecting that writing to ramdisk must be causing
> some interesting/expensive PV pagetable behavior?
> Or maybe somehow /dev/ram0 is getting routed through
> qemu?  Or ??
>   

I haven't looked at ramdisk, but I'm pretty sure there's nothing special
about accessing it.  The only thing I can think of is that if you're
using a 32bit highmem system then you may be being hit by lots of kmap
overhead.  But on a 64-bit system, AFAIK, it should just be memory copies.

    J